The present invention relates to apparatuses for continuous and efficient heat-treatment of semiconductor films upon thermally susceptible non-conducting substrates at a minimum thermal budget. More particularly, the invention relates to apparatuses for heat-treatment of preparing polycrystalline silicon thin-film transistors (poly-Si TFTs) and PN diodes on glass substrates for various applications of liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), and solar cells.
Liquid crystal displays (LCDs) and organic light emitting diodes (OLEDs) grow rapidly in the flat panel displays. In the present time, those display systems employ the active matrix circuit configuration using thin film transistors (TFTs). Fabrication of the TFTs on glass substrate is necessary in those applications.
TFT-LCDs typically uses the TFTs composing amorphous Si films as an active layer (i.e., a-Si TFT LCD). Recently, interests on the development of TFTs using polycrystalline silicon films instead of amorphous silicon films (i.e., poly-Si TFT LCD) is spurred because of their superior image resolution and merit of simultaneous integration of pixel area with peripheral drive circuits. In the area of OLEDs, uses of poly-Si TFTs provide evident advantages over a-Si TFTs, since the current derivability of poly-Si TFTs are substantially higher than that of a-Si TFTs, thus, leading to a higher operation performance.
The most formidable task for the fabrication of poly-Si devices on the commercially available glass substrates is a development of heat-treatment method that the glass substrate withstands at a minimum thermal budget. Glass is easily deformed when exposed to the temperature above 600xc2x0 C. for substantial length of time. The important heat-treatment steps that require high thermal budget for the fabrication of poly-Si devices include crystallization of amorphous Si films and electrical activation of implanted dopants for P (or N)-type junction. Those heat-treatments typically require high thermal budgets, unavoidably causing damage or distortion of glass.
Various methods for solving those problems have been developed. Those methods will be briefly reviewed with distinguishing areas of crystallization of amorphous Si and dopant activation.
(1) Heat-Treatments for Crystallization of Amorphous Si into Polycrystalline Si
A poly-Si film is typically obtained through deposition of an amorphous Si film by chemical vapor deposition method (CVD) and subsequent post-deposition crystallization heat-treatments.
Solid phase crystallization (SPC) is a popular method for crystallizing amorphous silicon. In this process, the amorphous silicon is subject to heat-treatments at temperatures approaching 600xc2x0 C. for a period of at least several hours. Typically, glass substrates are processed in a furnace having a resistive heater source. However, high thermal budget of this method leads to damage and/or distortion of used glass substrates.
Various crystallization methods exist for converting amorphous Si into polycrystalline Si at low temperatures without damaging glass. Important methods for this are excimer laser crystallization (ELC) and metal-induced crystallization (MIC).
The ELC method utilizes the nano-second laser pulse to melt and solidify the amorphous silicon into a crystalline form. However, this method has critical drawbacks for its use in mass production. The grain structure of poly-Si film through this process is extremely sensitive to the laser beam energy, so that the uniformity in grain structure and hence the device characteristics cannot be achieved. Also, the beam size of the laser is relatively small. The small beam size requires multiple laser passes or shots to complete the crystallization processes for large size glass. Since it is difficult to precisely control the laser, the multiple shots introduce non-uniformities into the crystallization process. Further, the surface of ELC poly-Si films is rough, which also degrades the performance of device. The ELC also has the problem of hydrogen eruption when deposited amorphous Si has high hydrogen contents, which is usually the case in the plasma enhanced chemical vapor deposition (PECVD). In order to prevent the hydrogen eruption, the heat-treatment for dehydrogenation should be required at high temperature (450xcx9c480xc2x0 C.) for long period ( greater than 2 hrs). In addition to the problems in the area of processes, the system of ELC process equipment is complicated, expensive, and hard to be maintained.
The MIC process involves addition of various metal elements such as Ni, Pd, Au, Ag, and Cu onto amorphous Si films in order to enhance the crystallization kinetics. Use of this method enhances the crystallization at low temperatures below 600xc2x0 C. This method, however, is limited by poor crystalline quality of poly-Si and metal contamination. The metal contamination causes a detrimental leakage current in the operation of poly-Si TFTs. Another problem of this method is a formation of metal silicides during the process. The presence of metal silicides leads to an undesirable residue problem during the following etching process step.
(2) Heat-Treatments for Dopant Activations
In addition to crystallization process, another heat-treatment process with high thermal budget is the dopant activation anneals. In order to form n type (or p type) regions such as source and drain regions of TFTs, dopants such as arsenic, phosphorus, or boron are implanted into Si films using ion implantation or plasma doping method. After doping of dopants, silicon is annealed for electrical activation (activation anneals). Similarly to a heat-treatment of crystallization, annealing is normally carried out in the furnace with a resistance heater source. This process requires high temperatures near 600xc2x0 C. and long duration time. Therefore, a new method for reducing thermal budget is needed and presented in the prior art. The excimer laser anneals (ELA) and rapid thermal anneals (RTA) are presented for those purposes.
The ELA uses the identical process mechanism with that of the ELC, that is, rapid re-melting and solidification of poly-Si with nano-second laser pulse. The problem which was found in the ELC for crystallization also exists here. The rapid thermal changes during the ELC process leads to an introduction of high thermal stress to the poly-Si films as well as the glass, and hence, the deterioration of device reliability.
The RTA method uses higher temperature but for short duration of time. Typically, the substrate is subjected to temperature approaching 700xcx9c1000xc2x0 C. during the RTA, however, the annealing process occurs relatively quickly, in minutes or seconds. Optical heating sources such as tungsten-halogen or Xe Arc lamps are often used as the RTA heat source. The problem of the RTA is that the photon radiation from those optical sources has the range of wavelength in which not only the silicon film but also the glass substrate is heated. Therefore, the glass is heated and damaged during the process.
Based upon the prior art, it is of great interest to develop apparatuses for enhancing the kinetics of crystallization and dopant activations for device fabrication on glass, and also to reduce the thermal budget required for those processes.
Accordingly, the objectives of the present invention are to solve the problems described above for once and all.
The present invention provides apparatuses for continuous and efficient heat-treatment of semiconductor films upon thermally susceptible non-conducting substrates at a minimum thermal budget. That is, the apparatuses for heat-treating the semiconductor films upon the thermally susceptible non-conducting substrates comprise:
(a) induction coils continuously forming an upper layer and a lower layer in such a way that the electromagnetic force can be generated parallel to the in-plane direction of semiconductor films, wherein materials consisting of thermally susceptible non-conducting substrates and semiconductor films deposited thereon (so called, xe2x80x9cheat-treatment materialsxe2x80x9d) can be moved into the space between said upper layer and said lower layer;
(b) magnetic cores covering the external surfaces of said upper layer and said lower layer, respectively; and,
(c) preheating member heating the heat-treatment materials for preparation of induction-heat prior to its movement into the space between said upper layer and said lower layer, wherein the semiconductor films of the heat-treatment materials is heated to the extent that said semiconductor films can be induction-heated at the minimum thermal budget acceptable for the use of substrates.
Representative examples of said semiconductor films are silicon films being amorphous silicon films or crystalline silicon films, and representative examples of said thermally susceptible non-conducting substrates are glass and plastic substrates.
According to the apparatuses of the present invention, the semiconductor films can be heat-treated continuously and efficiently without damaging the thermally susceptible substrates: e.g., crystallization of amorphous silicon films at the minimum thermal budget acceptable for the use of glass, enhancing kinetics of dopant activation at the minimum thermal budget acceptable for the use of glass.
Said silicon films of the heat-treatment materials are deposited on the glass substrate, in the form of either amorphous state crystallizing into polycrystalline in the case of crystallization heat-treatment, or polycrystalline state implanted by dopants (n or p type) in the case of dopant activation heat-treatment.
Said induction coils generate alternating magnetic flux on the semiconductor films by high frequency current applied thereto, thereby heating the semiconductor films.
In the configuration of forming the upper and lower layer with the induction coils, one of preferable examples is that the induction coil starting at an upper end is wound in the planar form by one or more than two rounds and then turns back to that upper end to form the upper layer, and subsequently the induction coil starting at a lower end corresponding to the upper end is wound in the planar form by one or more than two rounds and then turns back to that lower end to form the lower layer. In the upper and lower layers, winding directions and round numbers of the induction coil are the same with each other. Therefore, the alternating magnetic flux can be uniformly collimated in the direction perpendicular to the semiconductor films. The upper end being a starting point for forming the upper layer is located in the identical direction with the lower end being a starting point for forming the lower layer. Furthermore, when the induction coil which has formed the upper layer shifts to the starting point for forming the lower layer, the portion connecting both layers becomes to protrude sideways so as to prevent the contact of the induction coil itself. Accordingly, in the view of whole configuration, the upper and lower layers are separated from each other; however, both layers are connected at the side that the induction coil starts, but they are separated at the opposite side.
Preferably, in order to prevent overheating of the induction coils, a channel through which cooling water can be circulated is formed in the induction coil (for instance, water-cooling type cupper tube).
In any case, the number of winding layers of induction coils for forming the upper or lower layers may be more than two so at to generate further strengthened alternating magnetic flux.
Said magnetic cores are made of laminated metal core or ferrite core. Advantages of employing magnetic core are three fold. Firstly, it enhances the strength of magnetic field substantially with low induction power. Secondly, it makes the distribution of magnetic flux more uniform. Thirdly, it makes the flux distribution to be concentrated on the region of semiconductor films, which leads to more efficient heat-treatment and to prevention of undesired interference by the magnetic flux on the conducting components installed around them (for instance, chamber wall or external heater block).
One of examples for further efficiently showing these advantages is that the magnetic cores are configured to simultaneously cover the external surface and central portion of induction coil wound in the planar form to form the upper and lower layers. That is, at the cross sectional view perpendicular to the induction coil, two xe2x80x9cExe2x80x9d-type magnetic cores face each other on the imaginary central plane. Therefore, the outside part of the magnetic core covers the external surface of wound induction coil and the inside part of the magnetic core is inserted in the central portion of wound induction coil. In this configuration, the strongest alternating magnetic field is generated from the inside part of the magnetic core. While the heat-treatment materials move into the space formed by the upper and lower layers, in particular, between the two inside parts of the symmetrical magnetic cores, they can be induction-heated continuously and efficiently.
The length of gap between the upper and lower magnetic cores is not specially limited; however, if possible, it is desirable to narrow the length of gap for generation of the strong electromagnetic field. For example, the gap between the two magnetic cores may be designed to become less than the wide of the magnetic cores.
Said preheating members heat the semiconductor films to the extent that the thermally susceptible non-conducting substrates are not damaged and thus the semiconductor films can be induction-heated by the alternating magnetic flux from the induction coils. Generally, it has been understood that the induction heat used in the heat-treatment of conductive materials cannot be applied to the heat-treatment of semiconducting materials, because the latter requires the very strong induction power. However, the present inventors found that, when being heated to the specific temperature, even materials such as semiconductor films can be induction-heated with the small induction power, which is not a generally accepted idea. Accordingly, said preheating members make the semiconductor films acceptable to the condition for induction heating. According to the present invention, the preheating members heat the semiconductor films prior to its movement into the space, formed by the upper and lower layers of induction coil and the magnetic cores covering them, in which the high collimated alternating magnetic field is applied. Such preheating members ultimately heat the semiconductor films, and the type of preheating members may be manifold as the below.
In the first exemplary type, resistance heating sources are used to uniformly heat the atmosphere around the heat-treatment materials, which can minimize damage of the substrates by heating them wholly.
In the second exemplary type, heating plates are used being made of materials with a high resistance and good heat conductivity and being an electrically nonconductive such as AlN (Aluminum Nitride) or BN (Boron Nitride), in which the heat-treatment materials put on the heating plates are heated by the conduction heat from the heating plates
In the third exemplary type, heating plates are used being made of metal or graphite with a high conductivity, in which the heating plates are heated by the induction-heating from induction coils installed above, below or side the heating plates. That is, the heating plates are heated under the alternating magnetic field through a heating mechanism of eddy currents (i.e., induction heating), and then the heat-treatment materials are heated by the conduction heat from the heating plates.
Although the three types have been proposed as above, other types may be possible if they heat the semiconductor films to the specific temperature for preparation of induction-heating.
The upper limit of temperature in the preheating members should be kept lower than the distortion temperature of the thermally susceptible substrates; for example, in the case of glass, the temperature of the preheating members should be kept lower than about 600xc2x0 C. being the distortion temperature of glass. Meanwhile, the lower limit of temperature in the preheating members is not specifically limited, since the induction-heating become possible even at the low temperature when a strong high frequency current is applied. However, the application of the strong high frequency current requires enormous energy and the cost for making induction coils providing a very high induction magnetic field becomes very high, which is not desirable. Therefore, the temperature of the preheating members according to the present invention should be kept higher than about 200xc2x0 C., preferably 400xc2x0 C. For example, in the case that the semiconductor films on the non-conducting substrates are silicon films on glass substrates, the heating temperature of the preheating members is in the range of 200xcx9c600xc2x0 C., preferably 400xcx9c600xc2x0 C.
The apparatuses according to the present invention remarkably enhance the kinetics of crystallization of amorphous silicon, the solid phase crystallization (SPC) and the metal-induced crystallization (MIC). The apparatuses also remarkably enhance the kinetics of dopant activation of ion-implanted polycrystalline silicon.
The possible reason for the apparatuses according to the present invention to enhance the kinetics of said heat-treatment effects may be expressed as below. For simplicity, the semiconductor films are restricted to the silicon films and the thermally susceptible non-conducting substrates are restricted to the glass substrates, respectively.
Induction of alternating magnetic field inside the silicon films leads to generation of electromagnetic force (emf). Given assumption that the emf in the silicon films is the driving force for the kinetic enhancement, the Faraday""s Law (also see B. D. Cullity, xe2x80x9cIntroduction of Magnetic Materialsxe2x80x9d (Addison Wesley, Mass., 1972), P. 36 incorporated herein by reference) defines the strength of emf as follows:
EMF=10xe2x88x928Nd"PHgr"/dt volts
(Where N is the number of turns in the coil and d"PHgr"/dt is the rate of change of magnetic flux in the maxwell/sec unit.) Accordingly, the increase of kinetics depends on both the strength of magnetic flux and the alternating frequency. The alternating frequency is in the range of 1 kHzxcx9c10 MHz. When the alternating frequency is less than 1 kHz, the electromagnetic force becomes small, whereby it is difficult to lead the crystallization. When the alternating frequency is more than 10 MHz, it is difficult to make the induction coils and magnetic cores for generating the alternating magnetic field of large scale. The above range of alternating frequency may be changed more or less depending on the configuration of induction coils.
Even though mechanism for generation of emf to enhance the heat-treatment effects is not understood, a couple of reasons can be speculated.
First mechanism is a selective joule heating of silicon films. Amorphous or polycrystalline silicon has high resistivity values at room temperature, for instance, 106xcx9c1010 xcexa9-cm in the case of amorphous silicon. Thus, unless silicon is intentionally heated by external heat source, joule heating of silicon though said emf does not occur. However, when amorphous and polycrystalline Si are heated to elevated temperatures, their resistivities go down rapidly to the low values, for instance, 10xcx9c0.01 xcexa9-cm at 500xc2x0 C. Those resistivity values are similar to those of graphite (1xcx9c0.001 xcexa9-cm) and thus the induction-heating becomes possible. In spite of local heating of amorphous silicon under alternating magnetic flux, the glass substrate having high resistivity values (xcx9c1016 xcexa9-cm) is not heated by said alternating magnetic flux. Thus, the glass remains to be at low temperatures pre-set by the external heating.
Second mechanism is that said emf activates the movement of silicon atoms through a field effect functioning on the charged defects. It is known that point defects such as vacancies, interstitial atoms and impurities are electrically charged (negatively or positively) in the silicon atomic structure. Motion of those charged defects are significantly enhanced by the presence of electric field, which has been commonly reported in the academic publications (e.g., xe2x80x9cField-Enhanced Diffusionxe2x80x9d in silicon, see S. M. Sze xe2x80x9cVLSI Technologyxe2x80x9d (2nd ed. McGraw Hill, 1988), P. 287 incorporated herein by reference).
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed.